URITISH FERNS. 67 



A. adiantum nigrum, which, in some of its numerous varieties, it 

 nearly resembles. 



The root is black, very long, slender, and penetrating ; in the 

 fissures of rocks it runs to a great depth ; the rhizoma is brown, 

 tufted, and densely covered with bristle-hke scales ; the young 

 fronds appear in May, arrive at maturity in August, and last un- 

 injured throughout the winter : they are always fertile. 



The form of the frond is various ; in some situations it is of 

 erect growth, nearly linear, and simply pinnate ; the pinnae being 

 stalked and lobed : in this form it produces seed most abund- 

 antly, and the masses, when fully grown, are perfectly circular : 

 of the three entire fronds represented in the illustration (see the 

 preceding page), the one to the right is intended for this variety, 

 and the portions of fronds to the extreme right and left show the 

 situation of the veins, and mode of fructification : every part of 

 the frond is perfectly flat, and the entire plant rigid. A second 

 variety, of pendant growth, and larger size, is lanceolate in form ; 

 the pinnae are pinnate, the pinnulae stalked, serrated, and some- 

 what quadrate ; the fronds often reach a foot in length ; they 

 usually issue from dark holes or shaded spots, and the lower pair 

 of pinnae are often weak, bleached, and of small size; the surface 

 of the frond is generally flat, although sometimes slightly 

 inclined to concavity, and when this is the case, each pinnula 

 partakes more or less of the character : the middle frond of the 

 three represents this variety, and the detached pinnula immedi- 

 ately adjoining it shows the veins and incipient indusia. A 

 third variety is of nearly erect growth, but bends over at the 

 extremity, and the entire frond, together with each individual 

 pinnula, possesses such a rigid and inflexible convexity, that it is 

 next to impossible to flatten the plant by pressure : the frond to 

 the left is intended to represent this form, but the convexity is 

 not expressed. 



The lateral veins are branched, a branch running to the ex- 

 tremity of each serrature : the masses of thecae are attached near 

 the extremity of the veins, and somewhat alternately, one branch 

 bearing a mass, and the next being without one : each mass is 

 at first elongate and linear, and covered by a linear white 

 indusium; the indusium afterwards disappears, and the mass 

 becomes nearly circular. 



